The natural lens in the human eye is a transparent structure having a diameter of about 9 mm and a thickness of about 5 mm. It is generally lentil-shaped and is suspended behind the iris by zonular fibers which connect the lens to the ciliary body. A cataract is an opaque portion of the lens or of the anterior or posterior capsules which constitute a lens-enclosing bag. A cataract operation consists in removing the lens, and is designated as being an "intracapsular" operation when the capsule is removed together with the lens, and as being an "extracapsular" when the anterior capsule is removed together with the lens while the posterior capsule is left in place inside the eye.
The first intraocular lens was implanted by Ridley in 1949, and since then, various different types of artificial lens have been proposed. Particular mention may be made of U.S. Pat. Nos. 3,991,426 and 4,092,743.
One of the most critical problems with implanting and wearing an intraocular lens is the problem of fixing the implant inside the ocular cavity. For reasons of tolerance, instead of implanting lenses in the iris plane, lenses are now implanted either in the anterior chamber constituted by the cornea and the iris, or else in the posterior chamber constituted by the iris and the posterior lens capsule which is situated in front of the hyaloid membrane of the vitreous body.
In order to allow for a possible subsequent operation on a secondary cataract, proposals have been made in U.S. Pat. No. 4,244,860 to provide an annular lip including an opening on the posterior face of the optical part of an implant. It is thus possible, albeit difficult, to pass a sharp instrument through the opening to open the opaque posterior capsule without removing the implant. Unfortunately, the posterior capsule tends to invaginate itself inside the annulus against the posterior face of the implant.
Also, the Applicant has developed a new therapy using a pulsed laser beam emitted by a YAG rod operating in locked mode and operating by optical puncturing inside accurately determined regions of the eye without needing to open the eye (U.S. Pat. No. 4,309,998). It is thus possible using the apparatus described in that patent specification to open an opaque lens envelope in a fraction of a second without needing to use conventional ophthalmological surgical instruments, i.e. without opening the eye. Unfortunately, laser treatment performed behind an artificial lens may mark the lens. Since the capsule bears against the posterior face of the implant, there is a danger of the implant being permanently marked by the laser beam which is used to open the capsule.
The Applicant has already described, in U.S. patent application Ser. No. 540,796, an implant for mitigating this drawback by holding the posterior capsule away from the plane posterior face of the lens. In order to avoid the possibility of future laser marking on such an implant, the rear face of the lens is provided with spacer members serving to hold the posterior capsule away from the rear face of the lens by a distance of not less than 0.3 mm. Moreover, a particular biconvexity of the optics empeaches the YAG laser beam to be converged inside the body of the lens or at its posterior surface because as described in the U.S. Pat. No. 4,309,998 the true safety YAG implant distance is from 0.5 mm up to 1 mm and no spacers will provide this space without damaging the iris.
However, in some cases, it turns out that patients fitted with such implants can see the internal portions of the spacer members and they find this is disagreeable.
An object of the present invention is to mitigate this drawback.
Another object of the present invention is to provide an implant which ensures that adequate tension is applied to the posterior capsule.
Yet another object of the present invention is to provide an implant which makes it possible to operate completely safely on a secondary cataract.